The present invention relates to devices for detecting cardiac activity which utilize a plethysmograph; and more particularly to such devices for sensing the pulse from a patient during magnetic resonance imaging.
Magnetic resonance has been developed as an imaging method useful in diagnostic medicine. In magnetic resonance imaging (MRI) as is understood by those skilled in the art, a body being imaged is held within a uniform magnetic field oriented along a Z axis of a Cartesian coordinate system.
The net magnetizations of the nuclei in the body are excited to precession by means of a radio frequency (RF) pulse and the decaying precession of the spins produces the MRI signal. The amplitude of the MRI signal is dependent, among other factors, on the number of precession nuclei per volume within the image body termed the "spin density."
Magnetic gradient fields G.sub.x, G.sub.y and G.sub.z are applied along the X, Y and Z axes to impress position information onto the MRI signals through phase and frequency encoding. A set of MRI signals may then be "reconstructed" to produce an image. Each set of MRI signals is comprised of many "views," a view being defined as one or more MRI signal acquisitions made under the same X and Y gradient fields.
The length of time needed to acquire a sufficient number of views to reconstruct an image may be on the order of several minutes. Therefore, motion of the subject, including cardiac and respiratory motion, is inevitable. Such motion may produce blurring of the image or other image artifacts depending on the image reconstruction technique used. Two common image reconstruction techniques are associated with "spin warp" imaging as described in U.S. Pat. No. 5,051,903 or multiple angle projection reconstruction as disclosed in U.S. Pat. No. 4,471,306.
Object motion during the acquisition of an MRI image produces both blurring and "ghosts" in the phase encoded direction. Ghosts are particularly apparent when the motion is periodic, or nearly so. Both blurring and ghosts can be reduced if the data acquisition is synchronized with the functional cycle of the object. This method is known as gated MRI scanning, and its objective is to acquire MRI data at the same point during successive functional cycles so that the object "looks" the same in each view. Other techniques have been devised for removing the artifacts from the acquired data. For example, U.S. Pat. Nos. 4,580,219; 4,663,591 and 4,706,026 disclose methods by which a signal indicative of the functional cycle of the object can be used to process the image data to reduce the blurring and ghosting effects.
One of the periodic physiological functions that affects the MRI image is the cardiac cycle. Conventional sensors for an electrocardiograph or an arterial pulse plethysmograph cannot be utilized with MRI equipment. As noted, very intense magnetic fields are applied to the patient, some of which vary at radio frequencies. These electromagnetic fields can induce large electric currents in conventional electrocardiograph or plethysmograph sensors and cables, thereby subjecting the patient to electrical shock or burn hazards during the MRI scan. Therefore, the equipment to sense the periodic physiological functions of a patient undergoing an MRI scan must be specially designed to withstand the magnetic resonance environment and provide adequate patient safety even in the presence of large gradient magnetic and radio frequency fields. In addition, the MRI operator must pay careful attention to how the physiological sensing equipment is set up and where the patient interface cables are placed within the bore of the MRI magnet.
Even with proper protective measures in place, the resulting signal from the sensor often contains large noise spikes caused by the varying magnetic fields. The noise spikes may be in the same frequency band as the physiological signal and are difficult to filter out of the data. In addition, the cables and sensor are visible in the magnetic resonance image.